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158 lines
10 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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=====================================
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Virtual Memory Layout on RISC-V Linux
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=====================================
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:Author: Alexandre Ghiti <alex@ghiti.fr>
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:Date: 12 February 2021
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This document describes the virtual memory layout used by the RISC-V Linux
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Kernel.
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RISC-V Linux Kernel 32bit
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=========================
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RISC-V Linux Kernel SV32
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------------------------
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TODO
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RISC-V Linux Kernel 64bit
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=========================
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The RISC-V privileged architecture document states that the 64bit addresses
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"must have bits 63–48 all equal to bit 47, or else a page-fault exception will
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occur.": that splits the virtual address space into 2 halves separated by a very
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big hole, the lower half is where the userspace resides, the upper half is where
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the RISC-V Linux Kernel resides.
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RISC-V Linux Kernel SV39
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------------------------
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::
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========================================================================================================================
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Start addr | Offset | End addr | Size | VM area description
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========================================================================================================================
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| | | |
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0000000000000000 | 0 | 0000003fffffffff | 256 GB | user-space virtual memory, different per mm
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__________________|____________|__________________|_________|___________________________________________________________
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| | | |
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0000004000000000 | +256 GB | ffffffbfffffffff | ~16M TB | ... huge, almost 64 bits wide hole of non-canonical
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| | | | virtual memory addresses up to the -256 GB
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| | | | starting offset of kernel mappings.
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__________________|____________|__________________|_________|___________________________________________________________
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| Kernel-space virtual memory, shared between all processes:
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____________________________________________________________|___________________________________________________________
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| | | |
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ffffffc6fea00000 | -228 GB | ffffffc6feffffff | 6 MB | fixmap
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ffffffc6ff000000 | -228 GB | ffffffc6ffffffff | 16 MB | PCI io
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ffffffc700000000 | -228 GB | ffffffc7ffffffff | 4 GB | vmemmap
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ffffffc800000000 | -224 GB | ffffffd7ffffffff | 64 GB | vmalloc/ioremap space
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ffffffd800000000 | -160 GB | fffffff6ffffffff | 124 GB | direct mapping of all physical memory
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fffffff700000000 | -36 GB | fffffffeffffffff | 32 GB | kasan
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__________________|____________|__________________|_________|____________________________________________________________
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____________________________________________________________|____________________________________________________________
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ffffffff00000000 | -4 GB | ffffffff7fffffff | 2 GB | modules, BPF
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ffffffff80000000 | -2 GB | ffffffffffffffff | 2 GB | kernel
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__________________|____________|__________________|_________|____________________________________________________________
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RISC-V Linux Kernel SV48
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------------------------
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::
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========================================================================================================================
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Start addr | Offset | End addr | Size | VM area description
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========================================================================================================================
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0000000000000000 | 0 | 00007fffffffffff | 128 TB | user-space virtual memory, different per mm
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__________________|____________|__________________|_________|___________________________________________________________
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| | | |
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0000800000000000 | +128 TB | ffff7fffffffffff | ~16M TB | ... huge, almost 64 bits wide hole of non-canonical
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| | | | virtual memory addresses up to the -128 TB
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| | | | starting offset of kernel mappings.
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__________________|____________|__________________|_________|___________________________________________________________
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| Kernel-space virtual memory, shared between all processes:
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____________________________________________________________|___________________________________________________________
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| | | |
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ffff8d7ffea00000 | -114.5 TB | ffff8d7ffeffffff | 6 MB | fixmap
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ffff8d7fff000000 | -114.5 TB | ffff8d7fffffffff | 16 MB | PCI io
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ffff8d8000000000 | -114.5 TB | ffff8f7fffffffff | 2 TB | vmemmap
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ffff8f8000000000 | -112.5 TB | ffffaf7fffffffff | 32 TB | vmalloc/ioremap space
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ffffaf8000000000 | -80.5 TB | ffffef7fffffffff | 64 TB | direct mapping of all physical memory
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ffffef8000000000 | -16.5 TB | fffffffeffffffff | 16.5 TB | kasan
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__________________|____________|__________________|_________|____________________________________________________________
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| Identical layout to the 39-bit one from here on:
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____________________________________________________________|____________________________________________________________
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| | | |
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ffffffff00000000 | -4 GB | ffffffff7fffffff | 2 GB | modules, BPF
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ffffffff80000000 | -2 GB | ffffffffffffffff | 2 GB | kernel
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__________________|____________|__________________|_________|____________________________________________________________
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RISC-V Linux Kernel SV57
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------------------------
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::
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========================================================================================================================
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Start addr | Offset | End addr | Size | VM area description
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========================================================================================================================
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| | | |
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0000000000000000 | 0 | 00ffffffffffffff | 64 PB | user-space virtual memory, different per mm
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__________________|____________|__________________|_________|___________________________________________________________
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| | | |
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0100000000000000 | +64 PB | feffffffffffffff | ~16K PB | ... huge, almost 64 bits wide hole of non-canonical
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| | | | virtual memory addresses up to the -64 PB
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| | | | starting offset of kernel mappings.
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__________________|____________|__________________|_________|___________________________________________________________
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| Kernel-space virtual memory, shared between all processes:
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____________________________________________________________|___________________________________________________________
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| | | |
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ff1bfffffea00000 | -57 PB | ff1bfffffeffffff | 6 MB | fixmap
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ff1bffffff000000 | -57 PB | ff1bffffffffffff | 16 MB | PCI io
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ff1c000000000000 | -57 PB | ff1fffffffffffff | 1 PB | vmemmap
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ff20000000000000 | -56 PB | ff5fffffffffffff | 16 PB | vmalloc/ioremap space
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ff60000000000000 | -40 PB | ffdeffffffffffff | 32 PB | direct mapping of all physical memory
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ffdf000000000000 | -8 PB | fffffffeffffffff | 8 PB | kasan
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__________________|____________|__________________|_________|____________________________________________________________
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| Identical layout to the 39-bit one from here on:
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____________________________________________________________|____________________________________________________________
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| | | |
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ffffffff00000000 | -4 GB | ffffffff7fffffff | 2 GB | modules, BPF
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ffffffff80000000 | -2 GB | ffffffffffffffff | 2 GB | kernel
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__________________|____________|__________________|_________|____________________________________________________________
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Userspace VAs
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--------------------
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To maintain compatibility with software that relies on the VA space with a
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maximum of 48 bits the kernel will, by default, return virtual addresses to
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userspace from a 48-bit range (sv48). This default behavior is achieved by
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passing 0 into the hint address parameter of mmap. On CPUs with an address space
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smaller than sv48, the CPU maximum supported address space will be the default.
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Software can "opt-in" to receiving VAs from another VA space by providing
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a hint address to mmap. A hint address passed to mmap will cause the largest
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address space that fits entirely into the hint to be used, unless there is no
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space left in the address space. If there is no space available in the requested
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address space, an address in the next smallest available address space will be
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returned.
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For example, in order to obtain 48-bit VA space, a hint address greater than
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:code:`1 << 47` must be provided. Note that this is 47 due to sv48 userspace
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ending at :code:`1 << 47` and the addresses beyond this are reserved for the
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kernel. Similarly, to obtain 57-bit VA space addresses, a hint address greater
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than or equal to :code:`1 << 56` must be provided.
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